DE 197 07 518 C1 relates to a corrugated tube composed of thermoplastic polymer material for use as fluid line, with at least one polymer layer, having coherent, geometric external profiles separated from one another in the direction of the axis of the tube and defining, in succession in at least one angular region in an axial longitudinal direction, a corrugation on the curved surface of the tube, where the coherent, geometric external profiles have been formed in such a way that two approximately opposite generatrix lines on the curved surface of the tube are free from corrugations and these generatrix lines extend in the longitudinal direction of the tube.
WO 1999/0022171 A1 describes a folding bellows composed of a hose with a cross section that is in essence circular, where the hose has an external surface and an internal surface, and a large plurality of raised peripheral convolutes which are formed in the surface of the hose, where the inner surface of the hose defines the inner surface of the convolutes, and where the external surface of the hose defines the external surface of the convolutes, characterized in that at least one of the convolutes has been formed in such a way that a section of the convolute is in essence level with the surface of the hose, while the cross section, which is in essence circular, of the hose is retained.
WO 2002/002981 A1 describes a flexible pipe with a bellows which has a plurality of convolutes which have been formed in the wall of the tube and which run across an external surface of the tube, where the convolutes are separate in the axial direction of the tube, and where at least one of the convolutes has two opposite bending sections and, located between the bending sections, two restricted elongation sections, and where the height of the bending sections above the external surface of the tube is greater than the height of the restricted elongation sections above the external surface of the tube. The wall thickness of the tube described in WO 2002/002981 A1 is specifically uniform in the peripheral direction (WO 02/02981 A1, (D1), page 12, lines 12-14).
EP 1 233 223 A2 describes a tube with defined axial stiffness, manufactured from a material with at least one flexible bellows region formed in a manner similar to that of corrugations, where at least two regions of the bellows have been formed flat in an axial direction in order to reduce axial length change due to high internal pressure.
EP 1 176 351 A2 defines a tubular hose with a plurality of convolutes, which individually have alternating peaks and valleys. Each convolute has a height which is described by the difference of the radii between the corresponding peaks and valleys. The height within a convolute varies across the periphery of the tube, with a maximum height and a minimum height. The minimum height of the smallest or smallest-height region of a convolute corresponds to at least 10% of the maximum height of this convolute. In one embodiment mentioned in EP 1 176 351 A2, the maximum height of a convolute is at least twice the minimum height of this convolute.
A disadvantage of the prior-art embodiments is that the wall thickness ratio between the restricted elongation zones and bending zones cannot be varied. The wall thickness in the bending zones, and the prescribed geometry of the tube in the region of the convolutes, inevitably determines the wall thickness in the restricted elongation zones and thus the elasticity of the restricted elongation zones for particular relative movements of the two ends of the tube. For a given geometry, a change of elasticity in the restricted elongation zones can be brought about only via changes in the material or a change in the wall thickness of the entire tube. If the material and the fundamental wall thickness of the tube have been prescribed, the only remaining way of adjusting the flexibility of the component is to change its geometry, and this in turn requires a modification/change to the tooling used to produce the flexible tube or corrugated tube.
Concentrations of stress can also occur when internal pressure is applied in prior-art embodiments by virtue of the change in stiffness, in the transition zones between the convolutes and the straight region between two convolutes (see, for example, WO 2002/002981A1, page 6, lines 8-16) or there can be an adverse effect on the mechanical properties of the components, caused by notches (WO 2002/002981 A1, page 13, FIGS. 6A and 6B).
Because of the characteristic nature of the typical processing methods used for tubes, the internal surface of the flat convolutes (restricted elongation zones) can be almost straight, as shown in some of the figures in WO 2002/002981 A1, e.g. FIG. 6. FIG. 8D of the same prior art indeed shows a straight internal surface. The effect of these flat surfaces is that the stress placed on the material in these restricted elongation zones, for particular relative movements of the two ends of the tube in an axial longitudinal direction, is independent of the wall thickness.
Finally, if wall thickness in the restricted elongation zones is more uniform than in the remainder of the convolutes, the result is high stresses in these restricted elongation zones when internal pressure is applied, since most of the load is absorbed by these stiffer sections (see WO 2002/002981 A1, page 10, lines 6 to 25: “ . . . which design has the beneficial effects of providing a more uniform wall thickness in the restrained elongation section of the bellows”). When internal pressure is applied, the stress within the material depends on the projected pressurized cross section and on the wall thickness of the tube. It can therefore be advantageous to have greater wall thickness rather than uniform wall thickness, in particular in the restricted elongation zones.
It is therefore an object of the present invention to provide a flexible tube which withstands high internal pressure, preferably 3.2 bar absolute and more, and which moreover withstands this without, or with only very little, loss of flexural flexibility.
The difficulty here is the required combination of stiffness in an axial longitudinal direction when high internal pressure is applied, as mentioned above (this being a problem that could be solved by using a straight tube) with the flexural flexibility demanded (this being a problem which could be solved by using a tube with a conventional folding bellows). The advantage in relation to the longitudinal stiffness of a straight tube is attended by high bending resistance. The high flexibility of a tube with a folding bellows section of normal shape is attended by low resistance to longitudinal elongation when high internal pressure is applied.